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    <meta name="created" content="2018-10-23T06:18:10.521000000">
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      Gray Code Counter
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      <h1>
        Gray Code Counter
      </h1>
      <p>
        This orientation to the Logisim libraries concludes with a fairly sophisticated Gray code counter that allows the user to alter its current value using the Poke Tool and to place a label on the component using the Text Tool. It also customizes the icon that appears in the explorer, associated with the tool.
      </p>
      <h2>
        GrayCounter
      </h2>
      <pre>package com.cburch.gray;

import java.net.URL;

import javax.swing.ImageIcon;

import com.cburch.logisim.data.Attribute;
import com.cburch.logisim.data.BitWidth;
import com.cburch.logisim.data.Bounds;
import com.cburch.logisim.data.Direction;
import com.cburch.logisim.instance.Instance;
import com.cburch.logisim.instance.InstanceFactory;
import com.cburch.logisim.instance.InstancePainter;
import com.cburch.logisim.instance.InstanceState;
import com.cburch.logisim.instance.Port;
import com.cburch.logisim.instance.StdAttr;
import com.cburch.logisim.util.GraphicsUtil;
import com.cburch.logisim.util.StringUtil;

/** Manufactures a counter that iterates over Gray codes. This demonstrates
 * several additional features beyond the SimpleGrayCounter class. */
class GrayCounter extends InstanceFactory {
    public GrayCounter() {
        super("Gray Counter");
        setOffsetBounds(Bounds.create(-30, -15, 30, 30));
        setPorts(new Port[] {
                new Port(-30, 0, Port.INPUT, 1),
                new Port(  0, 0, Port.OUTPUT, StdAttr.WIDTH),
        });

        // We'll have width, label, and label font attributes. The latter two
        // attributes allow us to associate a label with the component (though
        // we'll also need configureNewInstance to configure the label's
        // location).
        setAttributes(
                new Attribute[] { StdAttr.WIDTH, StdAttr.LABEL, StdAttr.LABEL_FONT },
                new Object[] { BitWidth.create(4), "", StdAttr.DEFAULT_LABEL_FONT });

        // The following method invocation sets things up so that the instance's
        // state can be manipulated using the Poke Tool.
        setInstancePoker(CounterPoker.class);

        // These next two lines set it up so that the explorer window shows a
        // customized icon representing the component type. This should be a
        // 16x16 image.
        URL url = getClass().getClassLoader().getResource("com/cburch/gray/counter.gif");
        if(url != null) setIcon(new ImageIcon(url));
    }

    /** The configureNewInstance method is invoked every time a new instance
     * is created. In the superclass, the method doesn't do anything, since
     * the new instance is pretty thoroughly configured already by default. But
     * sometimes you need to do something particular to each instance, so you
     * would override the method. In this case, we need to set up the location
     * for its label. */
    protected void configureNewInstance(Instance instance) {
        Bounds bds = instance.getBounds();
        instance.setTextField(StdAttr.LABEL, StdAttr.LABEL_FONT,
                bds.getX() + bds.getWidth() / 2, bds.getY() - 3,
                GraphicsUtil.H_CENTER, GraphicsUtil.V_BASELINE);
    }

    public void propagate(InstanceState state) {
        // This is the same as with SimpleGrayCounter, except that we use the
        // StdAttr.WIDTH attribute to determine the bit width to work with.
        BitWidth width = state.getAttributeValue(StdAttr.WIDTH);
        CounterData cur = CounterData.get(state, width);
        boolean trigger = cur.updateClock(state.getPort(0));
        if(trigger) cur.setValue(GrayIncrementer.nextGray(cur.getValue()));
        state.setPort(1, cur.getValue(), 9);
    }

    public void paintInstance(InstancePainter painter) {
        // This is essentially the same as with SimpleGrayCounter, except for
        // the invocation of painter.drawLabel to make the label be drawn.
        painter.drawBounds();
        painter.drawClock(0, Direction.EAST);
        painter.drawPort(1);
        painter.drawLabel();

        if(painter.getShowState()) {
            BitWidth width = painter.getAttributeValue(StdAttr.WIDTH);
            CounterData state = CounterData.get(painter, width);
            Bounds bds = painter.getBounds();
            GraphicsUtil.drawCenteredText(painter.getGraphics(),
                    StringUtil.toHexString(width.getWidth(), state.getValue().toIntValue()),
                    bds.getX() + bds.getWidth() / 2,
                    bds.getY() + bds.getHeight() / 2);
        }
    }
}
</pre>
      <h2>
        CounterPoker
      </h2>
      <pre>package com.cburch.gray;

import java.awt.Color;
import java.awt.Graphics;
import java.awt.event.KeyEvent;
import java.awt.event.MouseEvent;

import com.cburch.logisim.data.BitWidth;
import com.cburch.logisim.data.Bounds;
import com.cburch.logisim.data.Value;
import com.cburch.logisim.instance.InstancePainter;
import com.cburch.logisim.instance.InstancePoker;
import com.cburch.logisim.instance.InstanceState;
import com.cburch.logisim.instance.StdAttr;

/** When the user clicks a counter using the Poke Tool, a CounterPoker object
 * is created, and that object will handle all user events. Note that
 * CounterPoker is a class specific to GrayCounter, and that it must be a
 * subclass of InstancePoker in the com.cburch.logisim.instance package. */
public class CounterPoker extends InstancePoker {
    public CounterPoker() {}

    /** Determines whether the location the mouse was pressed should result
     * in initiating a poke.
     */
    public boolean init(InstanceState state, MouseEvent e) {
        return state.getInstance().getBounds().contains(e.getX(), e.getY());
            // Anywhere in the main rectangle initiates the poke. The user might
            // have clicked within a label, but that will be outside the bounds.
    }

    /** Draws an indicator that the caret is being selected. Here, we'll draw
     * a red rectangle around the value. */
    public void paint(InstancePainter painter) {
        Bounds bds = painter.getBounds();
        BitWidth width = painter.getAttributeValue(StdAttr.WIDTH);
        int len = (width.getWidth() + 3) / 4;

        Graphics g = painter.getGraphics();
        g.setColor(Color.RED);
        int wid = 7 * len + 2; // width of caret rectangle
        int ht = 16; // height of caret rectangle
        g.drawRect(bds.getX() + (bds.getWidth() - wid) / 2,
                bds.getY() + (bds.getHeight() - ht) / 2, wid, ht);
        g.setColor(Color.BLACK);
    }

    /** Processes a key by just adding it onto the end of the current value. */
    public void keyTyped(InstanceState state, KeyEvent e) {
        // convert it to a hex digit; if it isn't a hex digit, abort.
        int val = Character.digit(e.getKeyChar(), 16);
        BitWidth width = state.getAttributeValue(StdAttr.WIDTH);
        if(val &lt; 0 || (val &amp; width.getMask()) != val) return;

        // compute the next value
        CounterData cur = CounterData.get(state, width);
        int newVal = (cur.getValue().toIntValue() * 16 + val) &amp; width.getMask();
        Value newValue = Value.createKnown(width, newVal);
        cur.setValue(newValue);
        state.fireInvalidated();

        // You might be tempted to propagate the value immediately here, using
        // state.setPort. However, the circuit may currently be propagating in
        // another thread, and invoking setPort directly could interfere with
        // that. Using fireInvalidated notifies the propagation thread to
        // invoke propagate on the counter at its next opportunity.
    }
}
</pre>
      <p>
        <b>Next:</b> <a href="guide.html">Guidelines</a>.
      </p>
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